A knowledge of how protein synthesis and catabolism are altered in the attempt of an organism to meet the nutritional crises brought by a dietary lack of protein or calories is important and imperative, particularly in the light of today's fears for the future. It is hoped that in the course of this investigation, we will gain a better understanding of the mechanism of the regulation of protein turnover. We will stress isotope methods which can give valid estimates of the rates of synthesis and degradation of pure proteins from liver and muscle, and which can then be used clinically. Ornithine aminotransferase is an adaptive liver enzyme with a rapid renewal rate, i.e., a high biological vulnerability. Cytochrome oxidase is a very constant enzyme of the mitochondria, common to both tissues. In muscle, aldolase is a sarcoplasmic protein and myosin A is a major constituent of the myofibril. These proteins will be isolated and the kinetics of their renewal determined. The rat will be used because it utilizes the liver as a primary source of amino acids under conditions of deprivation; the pig will be used because, like humans, skeletal muscle is mobilized as a primary protein source.